1. Field of the Invention
The present invention generally relates to a method of forming quantum wires for compound semiconductors used in optical communications, and more particularly to a method for forming quantum wires for compound semiconductors capable of forming a large number of quantum wires with quantum well.
2. Description of the Related Art
Multi-layer compound semiconductor structure can be grown into continuous single crystal layers wherein each single crystal layer has a different energy gap from its adjacent layers. For example, in case of a multi-layer structure of a GaAs layer having a narrower energy gap sandwiched between two AlGaAs layers having a wider energy gap, a spatial change arises in the conduction band and valence band of the GaAs layer. More specifically, at conduction band of the GaAs layer, electrons exist not at a normal conduction band level but at a quantum level of an potential well. If the quantum well is applied to a laser diode, the laser diode has the lower threshold current.
Recently, in order to enhance the efficiency of compound semiconductor devices having a quantum well, researches on quantum wire or quantum dot have been progressed. The quantum wires are formed by a selective epitaxial growth method on a GaAs substrate in which either one of groove or mesa is formed.
FIG. 1A through FIG. 1C are sectional views showing a conventional method of forming quantum wires.
As shown in FIG. 1A, a GaAs buffer layer 11 is formed on a GaAs substrate 10. On the GaAs buffer layer 11 are formed Al.sub.X Ga.sub.1-X As layers 12a, 12b, 12c, 12d and GaAs layers 13a, 13b, 13c, 13d in turn. Next, on the upper GaAs layer 13c is formed a photoresist film pattern 14 by photolithography.
As shown in FIG. 1B, the GaAs layers 13a, 13b, 13c, 13d and the Al.sub.X Ga.sub.1-X As layers 12a, 12b, 12c, 12d are mesa etched using the photoresist film pattern 14 as an etching mask. At this time, the lower Al.sub.X Ga.sub.1-X As layer 12a is etched to a selected thickness.
As shown in FIG. 1C, the photoresist film pattern 14 is removed by a well-known method. Next, a Al.sub.X Ga.sub.1-X As layer 15 is formed on the mesa etched portion by a selective epitaxial growth method, so that the Al.sub.X Ga.sub.1-X As layers 12a, 12b, 12c, 12d 15 surrounds the GaAs layers 13a, 13b, 13c, 13d whereby quantum wires are formed.
However, owing to the selective epitaxial growth method which has difficult growth conditions, the above described the conventional method of forming quantum wires has difficulty on manufacturing process. Further, in case the quantum wires are adapted to optical device, the optical device has the lower threshold current compared to the one having a quantum well since the active layer formed by the quantum wires has the smaller volume limited by the mesa or the groove. However, it is difficult to adapt the quantum wire in optical devices because of the weaken intensity of the laser. It is also difficult to produce a large number of quantum wires since they are formed by utilizing mesa or grooves.